Method and composition for hydroxylation of aromatic substrates

ABSTRACT

A method and composition are disclosed for the hydroxylation of aromatic substrates in the presence of oxygen, hydrogen, and a catalyst. In a preferred embodiment, benzene is oxidized to phenol in the presence of oxygen, a vanadium catalyst, and hydrogen. The method is economical, safe, and amenable to commercial scale-up.

BACKGROUND OF THE INVENTION

[0001] This invention relates to methods for the hydroxylation ofaromatic substrates. In particular, this invention relates to a methodfor producing hydroxyaromatic compounds by the oxidation of aromaticsubstrates in the presence of oxygen, hydrogen, and a catalyst. Theinvention also relates to catalyst compositions for effecting saidhydroxylation.

[0002] Phenol is among the most important industrial organic chemicalintermediates, being used for the manufacture of thermoplastics andother resins, dyestuffs, explosives, agrochemicals, and pharmaceuticals.It is particularly important in the manufacture of phenol-formaldehyderesins used in the construction, appliance, and automotive industries,and in the manufacture of bisphenol A for epoxy and polycarbonateresins.

[0003] Despite its industrial importance, prior art methods for theproduction of phenol are non-selective, multi-step, and/or expensive.For example, benzene may be alkylated to obtain cumene, which in turn isoxidized to form cumene hydroperoxide. The hydroperoxide is cleavedusing an acid catalyst to form phenol and acetone. Another industrialprocess using oxidation of toluene requires expensive startingmaterials. Older industrial processes such as the Raschig Hooker processrequire high energy input, and result in corrosive or difficult todispose of wastes.

[0004] More recent processes for the production of phenols include thehydroxylation of aromatic substrates using hydrogen peroxide in thepresence of a titanoaluminate molecular sieve, as disclosed in U.S. Pat.No. 5,233,097 to Nemeth et al., or in the presence of a hydrogenfluoride-carbon dioxide complex as disclosed in U.S. Pat. No. 3,453,332to Vesely et al. U.S. Pat. No. 5,110,995 further discloses hydroxylationof phenol or phenol derivatives in the presence of nitrous oxide andzeolite catalyst. A multi-step process requiring partial hydrogenationof benzene, separation of the reaction products, oxidation of some ofthe reaction products, dehydrogenation, and other steps is disclosed inU.S. Pat. No. 5,180,871 to Matsunaga et al. U.S. Pat. No. 5,001,280 toGubelmann et al., U.S. Pat. No. 5,110,995 to Kharitonov et al., and U.S.Pat. No. 5,756,861 to Panov et al. disclose oxidation of benzene tophenol by nitrous oxide in the presence of a zeolitic catalyst, withyields of up to about 16%.

[0005] While certain of these methods provide good yields, they stillsuffer from various drawbacks and disadvantages. In particular, nitrousoxide is expensive, and it is also a greenhouse gas that presentssignificant environmental concerns. Thus, despite the number of methodsavailable to synthesize hydroxyaromatic compounds, there still remains aneed for a process that is simple, high-yield, environmentally friendly,economical, and amenable to commercial scale-up.

SUMMARY OF THE INVENTION

[0006] The above-described drawbacks and disadvantages are alleviated bythe method described herein, which is a method of hydroxylating anaromatic substrate, which comprises reacting an aromatic substratehaving at least one active aromatic hydrogen in the presence of oxygen,hydrogen and a catalyst. The method is environmentally friendly,economical, safe, and amenable to commercial scale-up.

[0007] In another embodiment the invention comprises a catalystcomposition for hydroxylating an aromatic substrate having at least oneactive aromatic hydrogen, comprising oxygen, hydrogen, a vanadium,niobium, or tantalum precursor or mixture thereof, at least one anionicligand precursor, and at least one neutral, electron-donating ligandprecursor.

DETAILED DESCRIPTION OF THE INVENTION

[0008] The present method is directed to hydroxylation of aromaticsubstrates in the presence of oxygen, hydrogen, and a catalyst. Onepreferred embodiment comprises hydroxylation of benzene in the presenceof oxygen, hydrogen, and a vanadium catalyst.

[0009] One or more of a range of aromatic substrates may be hydroxylatedin the practice of this method. Preferably the aromatic substrate isbenzene, naphthalene, anthracene, phenanthrene, or the like, orsubstituted derivatives thereof. The substituents may be the same ordifferent. The number of substituents may vary, as long as at least oneactive aromatic hydrogen is available for substitution, where an activearomatic hydrogen is one capable of being replaced by hydroxyl toproduce a hydroxyaromatic compound. Benzene, for example, may have fromone to five substituents, which may the same or different.

[0010] Suitable substituents include one or more aryl groups, forexample phenyl, naphthyl, anthracyl, and phenanthryl. The arylsubstituents may themselves be substituted by various functional groups,providing that such functional groups do not interfere with thehydroxylation. Suitable functional groups include, but are not limitedto, alkyl groups as described below, carboxylic acids, carboxylic acidalkyl and aryl esters, aldehydes, hydroxyls, olefins, and alkyl and arylethers. Mixtures of different aryl groups and/or substituted aryl groupsas substituents are also within the scope of the invention.

[0011] Other suitable substituents include one or more alkyl groups,wherein the alkyl groups are straight- or branched-chain, or cyclic, andtypically have from one to twenty six carbons. Some illustrativenon-limiting examples of these alkyl groups include methyl, ethyl,propyl, isopropyl, butyl, tertiary-butyl, pentyl, neopentyl, hexyl,cyclobutyl, cyclopentyl, cyclohexyl, methylcyclohexyl, cycloheptyl.Exemplary alkyl-substituted benzenes include, but are not limited to,toluene, xylene, and cumene. The alkyl groups may themselves besubstituted by various functional groups, providing that such functionalgroups do not interfere with the hydroxylation. Suitable functionalgroups include, but are not limited to, aryl groups as described above,carboxylic acids, carboxylic acid alkyl and aryl esters, aldehydes,hydroxyls, olefins, and alkyl and aryl ethers. Mixtures of differentalkyl groups and/or substituted alkyl groups as substituents are alsowithin the scope of the invention.

[0012] Other suitable substituents include, but are not limited to, oneor more functional groups, providing that such functional groups do notinterfere with the hydroxylation. Suitable functional groups include,but are not limited to, carboxylic acids, carboxylic acid alkyl and arylesters, aldehydes, hydroxyls, olefins, and alkyl and aryl ethers.Mixtures of different functional groups as substituents are also withinthe scope of the invention. Mixtures of substituents comprisingcombinations of functional groups, aryl groups, alkyl groups and/ortheir functionalized derivatives are also within the scope of theinvention.

[0013] Preferred aromatic substrates are benzene, and benzenesubstituted by alkyl groups, aryl groups, alkyl ethers, aryl ethers, orcombinations thereof. Especially preferred are biphenyl, phenyl phenol,toluene, cumene, phenol, and para-cumyl phenol.

[0014] Molecular oxygen may serve as both oxidant and source of hydroxyloxygen in the present hydroxylation method. Hydrogen may serve as areductant. The compositional ratio between oxygen and hydrogen ispreferably outside the explosive range from the viewpoint of safety. Thehydroxylation advantageously proceeds in the presence of a mixture ofoxygen, hydrogen, and up to about 90% of at least one inert gas, e.g.,nitrogen, argon, helium and the like. A preferred hydrogen source ismolecular hydrogen, which may be used directly or in a mixture,especially, e.g., as a mixture with the oxygen source. A preferredoxygen and hydrogen source comprises air, or mixtures comprising thecomponents of air. The partial pressure of oxygen is preferably in therange from about 0.02 megaPascals (MPa) to about 7.1 MPa, and thepartial pressure of hydrogen is preferably in the range from about 0.002MPa to about 1.42 MPa. The absolute total pressure of the reaction iswithin the range of about 0.1 MPa to about 36 MPa, and preferably withinthe range of about 1 MPa to about 8 MPa.

[0015] Preferred catalysts are based on precursors which under thereaction conditions produce a catalyst effective in the hydroxylation ofan active aromatic hydrogen. Such precursors include precursors givingrise to a metal complex, such as a vanadium, niobium or tantalum complexor mixtures thereof; precursors giving rise to an anionic ligand;precursors giving rise to a neutral, electron-donating ligand, andprecursors comprising a combination of vanadium, niobium or tantalumwith either an anionic ligand or a neutral, electron-donating ligand, orboth. The anionic and/or neutral, electron-donating ligands may bepresent in the same molecule, for example as bidentate or tridentateligands.

[0016] Suitable metal precursors include, but are not limited to, theoxides or the alkali metal salts of vanadium, niobium, or tantalum, forexample sodium metavanadate; substituted oxides of vanadium, niobium andtantalum, for example VO(acetylacetonate)₂ and VO(picolinate)₂, andalcoholates such as tantalum trisethoxide and niobium trisethoxide.Mixtures of metal precursors are also within the scope of the invention.In particular, mixtures of precursors containing either the same ordifferent metals are suitable.

[0017] Suitable anionic ligand precursors include, but are not limitedto, halides, carboxylic acids and/or their alkali metal or other salts,for example, sodium acetate, trifluoroacetate, beta-diketonates,acetylacetonate, propionate, butyrate, benzoate, or their correspondingacids; carboxylic acids and/or their alkali metal or other salts in aposition alpha to a heteroaromatic nitrogen atom, such as, but notlimited to, picolinic acid and substituted picolinic acids; picolinateand substituted picolinates, and their corresponding N-oxides. Suitablesubstituents for picolinic acid and picolinate include, but are notlimited to, carboxylic acid, carboxylate, halogen, alkyl, heteroaryl,and aryl. Suitable beta-diketonates include those known in the art asligands for the metal precursors of the present invention. Examples ofbetadiketones (from which beta-diketonates are derived) include, but arenot limited to, acetylacetone, benzoylacetone, dibenzoylmethane,diisobutyrylmethane, 2,2-dimethylheptane-3,5-dione,2,2,6-trimethylheptane-3,5-dione, dipivaloylmethane,trifluoroacetylacetone, hexafluoroacetylacetone,benzoyltrifluoroacetone, pivaloyltrifluoroacetone, heptafluorodimethyloctanedione, octafluorohexanedione,decafluoroheptanedione, 4,4,4-trifluoro-1-phenyl-1,3-butanedione,2-furoyltrifluoroacetone, 2-theonyltrifluoroacetone,3-chloro-2,4-pentanedione, 3-ethyl-2,4-pentanedione,3-methyl-2,4-pentanedione, methyl 4-acetyl-5-oxohexanoate. Mixtures ofanionic ligand precursors are also within the scope of the invention.Metal complexes of the anionic ligands are also usable, e.g.,VO(acetylacetonate)₂ and VO(picolinate)₂.

[0018] Suitable neutral, electron-donating ligand precursors include,but are not limited to, water; acetonitrile; nitrogen in aheteroaromatic ring, such as, but not limited to, pyridine, substitutedpyridines, picolinic acid or substituted picolinic acids; alcohols;hydroxyaromatic compounds; phenol; substituted phenols; ethers; furan;tetrahydrofuran; phosphines; amines; amides; ketones; esters; Schiffbases; or imides. Mixtures of neutral, electron-donating ligandprecursors are also within the scope of the invention.

[0019] The above precursors may be supplied to the solution separatelyor as metal complexes with at least one ligand. For example, onepreferred formulation comprises the combination at low pH (e.g., lessthan about 4, and preferably less than about 3) of sodium metavanadate,a carboxylic acid, and a compound containing heteroaromatic nitrogen,e.g., picolinic acid, substituted picolinic acids, pyridine, substitutedpyridines, or their corresponding N-oxides. Another preferredformulation comprises the combination of VO(acetylacetonate)₂ with acompound containing heteroaromatic nitrogen, e.g., a pyridyl compound.Still another combination comprises the combination of VO(picolinate)₂with a carboxylate and/or a compound containing heteroaromatic nitrogen,e.g., a pyridyl compound. In each of the above formulations, thecatalyst is formed in solution from a vanadium, niobium, or tantalumprecursor; a carboxylic acid precursor (which may be in the form of acarboxylic acid or acid salt, or which may also function as the metalprecursor); and a precursor compound containing heteroaromatic nitrogen,e.g., a pyridyl precursor (which may be in the form of the pyridylcompound itself, or which may also function as the metal precursor).

[0020] The stoichiometric ratio of the anionic ligand precursor to metal(i.e. vanadium, niobium, or tantalum, or mixture thereof) in thecomposition and stoichiometric ratio of the neutral, electron-donatingligand precursor to metal in the composition are not particularlylimited so long as there is a sufficient molar quantity of anionicligand and of neutral, electron-donating ligand to satisfy the vacantvalency sites on the metal in the active catalyst species effective inthe hydroxylation of an aromatic compound having at least one activearomatic hydrogen. In addition, the quantities of anionic ligand andneutral, electron-donating ligand are preferably not such that theyinterfere either with the hydroxylation reaction itself or with theisolation or purification of the product mixture, or with the recoveryand reuse of catalyst components (such as metal).

[0021] When a ligand precursor is also a hydroxyaromatic compoundproduced by the reaction, then the stoichiometric ratio of ligandprecursor to metal precursor may be directly related to the turnovernumber of the reaction, which is the yield of moles of product per molesof metal (or mixture of metals). The turnover number of the reactiondetermines the moles of hydroxyaromatic compounds produced. For optimumefficiency the turnover number is desired to be as high as possible.Preferred turnover numbers for the present invention are greater than 1,more preferably greater than about 10, and most preferably greater thanabout 50. Typically turnover numbers may be between about 5 and about50.

[0022] In preferred embodiments of the present invention thestoichiometric ratio of both the anionic ligand precursor to metal andthe neutral, electron-donating ligand precursor to metal in thecomposition are about 500-2:1, more preferably about 100-2:1, and stillmore preferably about 50-2:1. When the catalyst composition comprisesmetal precursor (or mixture of metal precursors) in which the metal issupplied in the form of, for example, a complex with either the anionicligand precursor, or the neutral, electron-donating ligand precursor, orboth, then the stoichiometric ratio of ligand precursor to metal isessentially 2:1, as for example in VO(acetylacetonate)₂ and inVO(picolinate)₂. It is also contemplated that additional, uncomplexedanionic ligand precursor, or uncomplexed neutral, electron-donatingligand precursor, or both, may be added to the reaction mixture when themetal is supplied in the form of a complex with either the anionicligand precursor, or the neutral, electron-donating ligand precursor, orboth.

[0023] Without being bound by theory, it is hypothesized that suitablecatalyst precursor combinations may give rise in the presence ofmolecular oxygen or a molecular oxygen precursor to catalysts having thegeneral structure

MO(O₂)(L¹)_(n)(L²)_(m)

[0024] wherein M is a metal such as vanadium, niobium or tantalum; n isan integer from 0 to 1; m is an integer from 1 to 3; L¹ is an anionic,mono- or bi-dentate ligand; and L² is a neutral, electron-donatingligand. Suitable anionic ligands include, but are not limited to,halides or the conjugate base of a carboxylic acid, for example,acetate, trifluoroacetate, beta-diketonates, acetylacetonate,propionate, butyrate, benzoate, and the conjugate base of a carboxylicacid in a position alpha to a heteroaromatic nitrogen atom, such as, butnot limited to, picolinate, and substituted picolinates, and theircorresponding N-oxides. Suitable neutral, electron-donating ligandsinclude, but are not limited to, water; acetonitrile; nitrogen in aheteroaromatic ring, such as, but not limited to, pyridine, pyridyl,picolinic acid or a substituted picolinic acid; alcohols;hydroxyaromatic compounds; phenol; substituted phenols; ethers; furan;tetrahydrofuran; phosphines; amine; amides; ketones; esters; Schiffbases; or imides.

[0025] The catalyst is present in an effective amount, which is readilydetermined empirically by one of ordinary skill in the art, depending onthe starting aromatic substrate, the desired reaction rate, the cost ofthe catalyst, and like considerations. Generally, the catalyst will bepresent in amounts of up to about 10 mole percent of the aromaticsubstrate.

[0026] The reaction temperature is generally within the range of about25° C. to about 200° C., preferably in the range of about 40° to about150° C. Although the reaction time depends upon reaction conditions, thereaction time is generally several seconds to several hours.

[0027] Although the reaction may be run neat in benzene, toluene, orother aromatic substrate, at least one inert solvent may also be usedwhere desirable to provide at least some degree of miscibility ormicrohomogeneity with respect to the catalyst, the aromatic substrate,oxygen and/or hydrogen. Solvents which enhance solubility and/orreactivity of the reactants are especially desirable, but the solventwill optimally solubilize, at least in part, the aromatic substrate, thecatalyst, and oxygen and/or hydrogen without significantly decreasingthe utilization efficiency of the catalyst. Exemplary solvents include,but are not limited to, acetonitrile, fluorinated hydrocarbons, freons,chloroform, dichloromethane, carbon tetrachloride, or combinationsthereof.

[0028] Hydroxylation may be practiced either in a batch,semi-continuous, or continuous process. In a batch reaction catalyst andligands are dissolved in the aromatic substrate or substrate/solventmixture, preferably under an inert atmosphere, and a gaseous mixturecomprising oxygen, hydrogen, and at least one inert gas is introducedinto the reaction vessel. Although not necessary, it is preferred thatthe gas mixture be sparged or vigorously mixed with the reaction liquorin order to enhance transport into the liquor and thus increase reactionrate. In this instance, the use of a homogenous feedstock isadvantageous in ensuring adequate contact between the catalyst and thearomatic substrate. The hydroxyaromatic compound or other productsproduced by the method of this invention may be separated and isolatedby conventional techniques.

[0029] The following Examples are provided by way of example only, andshould not be read to limit the scope of the invention.

EXAMPLE 1

[0030] 0.01 grams (g) of VO(acetylacetonate)₂, 0.02 g of picolinic acid,and 50 milliliters (mL) of benzene were added to a stainless steel bomb.The bomb was sealed with a cap containing a gas-sparging stir shaft andreactor cooling coils. The reactor was then brought to 100° C. withstirring, and pressurized with 2.1% hydrogen gas in air at 6.9 MPa.Stirring was continued at this temperature and pressure for about 18hours. The reaction was then cooled, and analysis by gas chromatographyindicated the presence of 0.012 g of phenol and no other reactionproducts, indicating a turnover number (yield of moles of product permoles of catalyst) of 3.5.

EXAMPLE 2

[0031] VO(picolinate)₂ or catalyst precursors which in solution produceVO(O₂)(picolinate)(L)_(n), (as described above) and benzene are added toa stainless steel bomb. The bomb is sealed with a cap containinggas-sparging stir shaft and reactor cooling coils. The reactor is thenbrought to reaction temperature (approximately 100° C.) with stirring,and pressurized with a mixture of hydrogen in air (approximately 6.9 MPaof 2.1% hydrogen gas). Stirring is continued at this temperature andpressure until no more gas uptake is observed. The reaction is thencooled, and analyzed by gas chromatography to show the presence ofphenol in benzene.

[0032] While preferred embodiments have been shown and described,various modifications and substitutions may be made thereto withoutdeparting from the spirit and scope of the invention. Accordingly, it isto be understood that the present invention has been described by way ofillustration and not limitation.

1. A method of hydroxylating an aromatic substrate, which comprisesreacting an aromatic substrate having at least one active aromatichydrogen in the presence of oxygen, hydrogen and a catalyst.
 2. Themethod of claim 1 , wherein the aromatic substrate is selected from thegroup consisting of benzene, naphthalene, anthracene, phenanthrene, andderivatives of the foregoing having one or more substituents.
 3. Themethod of claim 2 , wherein the substituents are selected from the groupconsisting of aryl groups, alkyl groups, functional groups, andcombinations thereof, wherein the functional groups are carboxylicacids, carboxylic acid alkyl esters, carboxylic acid aryl esters,aldehydes, hydroxyls, olefins, alkyl ethers, or aryl ethers.
 4. Themethod of claim 3 , wherein the substituents are substituted by one ormore moieties selected from the group consisting of aryl groups, alkylgroups, functional groups, or combinations thereof, wherein thefunctional groups are carboxylic acids, carboxylic acid alkyl esters,carboxylic acid aryl esters, aldehydes, hydroxyls, olefins, alkylethers, or aryl ethers.
 5. The method of claim 2 , wherein the aromaticsubstrate is benzene, or benzene substituted by at least one alkylgroup, aryl group, alkyl ether, aryl ether, hydroxyl, or combinationsthereof.
 6. The method of claim 5 , wherein the aromatic substrate isbenzene, biphenyl, phenyl phenol, toluene, cumene, phenol, or para-cumylphenol.
 7. The method of claim 2 , wherein the aromatic substrate isbenzene.
 8. The method of claim 1 , wherein the oxygen and hydrogen areprovided as a mixture of oxygen and hydrogen with at least one inertgas, or as a mixture of hydrogen with air.
 9. The method of claim 8 ,wherein the mixture of oxygen and hydrogen in inert gas comprises up toabout 90% inert gas.
 10. The method of claim 1 , wherein the oxygen andhydrogen are provided at a pressure between about 0.1 MPa and about 36MPa.
 11. The method of claim 1 , wherein the catalyst is formed insolution from a vanadium, niobium, or tantalum precursor or mixturethereof; at least one anionic ligand precursor; and at least oneneutral, electron-donating ligand precursor.
 12. The method of claim 11, wherein the stoichiometric ratio of anionic ligand precursor tovanadium, niobium, or tantalum, or mixture thereof, and thestoichiometric ratio of neutral, electron-donating ligand precursor tovanadium, niobium, or tantalum, or mixture thereof are each about500-2:1.
 13. The method of claim 11 , wherein the stoichiometric ratioof anionic ligand precursor to vanadium, niobium, or tantalum, ormixture thereof, and the stoichiometric ratio of neutral,electron-donating ligand precursor to vanadium, niobium, or tantalum, ormixture thereof are each about 100-2:1.
 14. The method of claim 11 ,wherein the stoichiometric ratio of anionic ligand precursor tovanadium, niobium, or tantalum, or mixture thereof, and thestoichiometric ratio of neutral, electron-donating ligand precursor tovanadium, niobium, or tantalum, or mixture thereof are each about50-2:1.
 15. The method of claim 11 , wherein the anionic ligandprecursor is at least one member selected from the group consisting ofhalides, carboxylic acids, acetic acid, trifluoroacetic acid, propionicacid, butyric acid, benzoic acid, beta-diketones, acetylacetone,conjugate bases of carboxylic acids, acetate, trifluoroacetate,propionate, butyrate, benzoate, beta-diketonates, acetylacetonate,carboxylic acids in a position alpha to a heteroaromatic nitrogen atom,picolinic acid, substituted picolinic acids, picolinic acid N-oxide,substituted picolinic acid N-oxides, conjugate bases of carboxylic acidsin a position alpha to a heteroaromatic nitrogen atom, picolinate,substituted picolinates, picolinate N-oxide, and substituted picolinateN-oxides; and the neutral ligand precursor is at least one memberselected from the group consisting of water, acetonitrile, nitrogen in aheteroaromatic ring, pyridine, substituted pyridines, picolinic acid,substituted picolinic acids, alcohols, hydroxyaromatic compounds,phenol, substituted phenols, ethers, furan, tetrahydrofuran, phosphines,amines, amides, ketones, esters, Schiff bases, and imides.
 16. Themethod of claim 11 , wherein the catalyst is formed in solution from avanadium, niobium, or tantalum precursor, or mixture thereof; acarboxylate precursor; and a pyridyl precursor.
 17. The method of claim11 , wherein the catalyst is formed in solution from a combination ofpicolinic acid, and at least one of sodium metavanadate, VO(picolinate)₂or VO(acetylacetonate)₂.
 18. The method of claim 1 , wherein a catalystin solution has the formula MO(O₂)(L¹)_(n)(L²)_(m), wherein M isvanadium, niobium or tantalum, n is an integer from 0 to 1, m is aninteger from 1 to 3, L¹ is an anionic, mono- or bi-dentate ligand, andL² is a neutral, electron-donating ligand.
 19. The method of claim 18 ,wherein L¹ is at least one member selected from the group consisting ofhalides, conjugate bases of carboxylic acids, acetate, trifluoroacetate,beta-diketonates, acetylacetonate, propionate, butyrate, benzoate,conjugate bases of carboxylic acids in a position alpha to aheteroaromatic nitrogen atom, picolinate, substituted picolinates,picolinate N-oxide, and substituted picolinate N-oxides; and L² is atleast one member selected from the group consisting of water,acetonitrile, nitrogen in a heteroaromatic ring, pyridine, substitutedpyridines, picolinic acid, substituted picolinic acids, alcohols,hydroxyaromatic compounds, phenol, substituted phenols, ethers, fu ran,tetrahydrofuran, phosphines, amines, amides, ketones, esters, Schiffbases, and imides.
 20. A method of hydroxylating an aromatic substrate,comprising reacting the aromatic substrate in the presence of oxygen,hydrogen, and an effective amount of a catalyst formed from vanadium,niobium, or tantalum precursors, an anionic, mono- or bi-dentate ligandprecursor, and a neutral, electron-donating ligand precursor.
 21. Themethod of claim 20 , wherein the aromatic substrate is benzene,naphthalene, anthracene, phenanthrene, or the foregoing substituted byat least one alkyl group, aryl group, hydroxyl, alkyl ether, aryl ether,or combinations thereof.
 22. A method of making phenol from benzene,comprising reacting benzene in the presence of oxygen, hydrogen, and aneffective amount of a catalyst formed from vanadium, niobium, ortantalum precursors, an anionic, mono- or bi-dentate ligand precursor,and a neutral, electron-donating ligand precursor.
 23. A composition forhydroxylating an aromatic substrate having at least one active aromatichydrogen, comprising oxygen, hydrogen, a vanadium, niobium, or tantalumprecursor or mixture thereof, at least one anionic ligand precursor, andat least one neutral, electron-donating ligand precursor.
 24. Thecomposition of claim 23 , further comprising at least one inert gas. 25.The composition of claim 23 , wherein the stoichiometric ratio ofanionic ligand precursor to vanadium, niobium, or tantalum, or mixturethereof, and the stoichiometric ratio of neutral, electron-donatingligand precursor to vanadium, niobium, or tantalum, or mixture thereofare each about 500-2:1.
 26. The composition of claim 23 , wherein thestoichiometric ratio of anionic ligand precursor to vanadium, niobium,or tantalum, or mixture thereof, and the stoichiometric ratio ofneutral, electron-donating ligand precursor to vanadium, niobium, ortantalum, or mixture thereof are each about 100-2:1.
 27. The compositionof claim 23 , wherein the stoichiometric ratio of anionic ligandprecursor to vanadium, niobium, or tantalum, or mixture thereof, and thestoichiometric ratio of neutral, electron-donating ligand precursor tovanadium, niobium, or tantalum, or mixture thereof are each about50-2:1.
 28. The composition of claim 23 , wherein the anionic ligandprecursor is at least one member selected from the group consisting ofhalides, carboxylic acids, acetic acid, trifluoroacetic acid, propionicacid, butyric acid, benzoic acid, beta-diketones, acetylacetone,conjugate bases of carboxylic acids, acetate, trifluoroacetate,propionate, butyrate, benzoate, beta-diketonates, acetylacetonate,carboxylic acids in a position alpha to a heteroaromatic nitrogen atom,picolinic acid, substituted picolinic acids, picolinic acid N-oxide,substituted picolinic acid N-oxides, conjugate bases of carboxylic acidsin a position alpha to a heteroaromatic nitrogen atom, picolinate,substituted picolinates, picolinate N-oxide, and substituted picolinateN-oxides; and the neutral ligand precursor is at least one memberselected from the group consisting of water, acetonitrile, nitrogen in aheteroaromatic ring, pyridine, substituted pyridines, picolinic acid,substituted picolinic acids, alcohols, hydroxyaromatic compounds,phenol, substituted phenols, ethers, fu ran, tetrahydrofuran,phosphines, amines, am ides, ketones, esters, Schiff bases, and imides.29. The composition of claim 23 , wherein the catalyst is formed insolution from a vanadium, niobium, or tantalum precursor, or mixturethereof; a carboxylate precursor; and a pyridyl precursor.
 30. Thecomposition of claim 29 , wherein the catalyst is formed in solutionfrom a combination of picolinic acid and at least one of sodiummetavanadate, VO(picolinate)₂ or VO(acetylacetonate)₂.
 31. Thecomposition of claim 23 , wherein a catalyst in solution has the formulaMO(O₂)(L¹)_(n)(L²)_(m), wherein M is vanadium, niobium or tantalum, n isan integer from 0 to 1, m is an integer from 1 to 3, L¹ is an anionic,mono- or bi-dentate ligand, and L² is a neutral, electron-donatingligand.
 32. The composition of claim 31 , wherein L¹ is at least onemember selected from the group consisting of halides, conjugate bases ofcarboxylic acids, acetate, trifluoroacetate, beta-diketonates,acetylacetonate, propionate, butyrate, benzoate, conjugate bases ofcarboxylic acids in a position alpha to a heteroaromatic nitrogen atom,picolinate, substituted picolinates, picolinate N-oxide, and substitutedpicolinate N-oxides; and L² is at least one member selected from thegroup consisting of water, acetonitrile, nitrogen in a heteroaromaticring, pyridine, substituted pyridines, picolinic acid, substitutedpicolinic acids, alcohols, hydroxyaromatic compounds, phenol,substituted phenols, ethers, furan, tetrahydrofuran, phosphines, amines,amides, ketones, esters, Schiff bases, and imides.
 33. The compositionof claim 23 , wherein the aromatic substrate is benzene, naphthalene,anthracene, phenanthrene, or the foregoing substituted by at least onealkyl group, aryl group, hydroxyl, alkyl ether, aryl ether, orcombinations thereof.
 34. A composition for making phenol from benzene,comprising oxygen, hydrogen, a vanadium, niobium, or tantalum precursoror mixture thereof, at least one anionic ligand precursor, and at leastone neutral, electron-donating ligand precursor.
 35. The composition ofclaim 34 , further comprising at least one inert gas.